Starting with macOS 15 Sequoia, Screens Assist (formerly Screens Express) will require permission to detect local connections:
If this permission is not granted, Screens Assist will be unable to open a port on your router:
To ensure Screens Assist has the necessary permissions, go to System Settings > Privacy & Security > Local Network, and make sure that Screens Assist (or Screens Express) is enabled:
Once allowed, the app should be able to open a port on your router and function properly.